R⼀S标记法中基团的优先顺序如何判断?要具体的!

2025-03-16 00:26:51
推荐回答(2个)
回答1:

先按原子序数大小排序,原子序数大的优先,同位素中原子量重的优先;如果第一次比较原子序数相同,按外推法比较,原子序数总和大的优先。

顺序:-COOH>-SO3H>-COOR>-COX>-CN>-CHO>>C=O>-OH(醇)>-OH(酚)>-SH>-NH2>-OR>C=C>-C≡C->(-R>-X>-NO2)

原子:原子序数大的排在前面,同位素质量数大的优先。几种常见原子的优先次序为:I>Br>Cl>S>P>O>N>C>H

饱和基团:如果第一个原子序数相同,则比较第二个原子的原子序数,依次类推。常见的烃基优先次序为:(CH3)3C->(CH3)2CH->CH3CH2->CH3-

在Fischer投影式中

当d基团出现在竖立键上时,由于竖立键是背向纸面,d基团刚好是位于远离观察者的方向,此时如果基团次序为a>b>c>d时,如果待标记分子的Fischer投影式中d基团是在横键上,则因这个是伸出于前方而不在远离观察者的位置上。

此时依次轮看a,b,c时,如果是顺时针方向旋转的,则代表的是R构型,反时针方向则代表S构型。这于d在竖立键上时的结论正好相反。

以上内容参考:百度百科-R-S系统命名法

回答2:

次序规则

(1)原子:原子序数大的排在前面,同位素质量数大的优先。几种常见原子的优先次序为:I>Br>Cl>S>P>O>N>C>H
(2)饱和基团:如果第一个原子序数相同,则比较第二个原子的原子序数,依次类推。常见的烃基优先次序为:(CH3)3C->(CH3)2CH->CH3CH2->CH3-
(3)不饱和基团:可看作是与两个或三个相同的原子相连。不饱和烃基的优先次序为:
-C≡CH>-CH=CH2>(CH3)2CH-
(4)若与双键碳原子相连的基团互为顺反异构时,Z型先于E型。

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